Deallocate the XTransferable object async using AsyncCallback
(that uses Application::PostUserEvent) which executes the
callback in a thread-safe way on the main thread. This avoids
a deadlock at deallocation so that the XTransferable.

Modify AsyncCallback to not hold the SolarMutexGuard because
Application::PostUserEvent is considered thread-safe.

Document Application::PostUserEvent thread-safety

Change-Id: I4237a1cf380e8be66b3eefc393a58bb4853bf4e1
Reviewed-on: https://gerrit.libreoffice.org/31126Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>